The present invention relates to a contact device with a plunger which moves longitudinally in a housing against the force of a pressure spring and has one or several recesses, wherein each recess accommodates a contact pressure spring and a contact bridge which extends outwardly beyond both sides of the recess and has movable contact pieces cooperating with the immovable contact pieces so as to form a contact breaker and/or a contact maker. There is further provided a member which cooperates with the contact bridge to force the opening of adhering or welded contacts between the movable and the immovable contact pieces.
Contact devices of the above-mentioned general type are known in the art and are used in connection with different constructions and for different application purposes. They are used for example in huge numbers as end switches. These contact devices have a common feature namely that they are provided with a plunger which is actuated manually or by a drive and serves simultaneously as a contact bridge carrier. In many switching devices of this type, only one contact bridge is available, while in other switching devices several contact bridges are arranged over one another and/or near one another. On the grounds of the switching safety the actuation path of the plunger or contact bridge carrier must be greater than the working stroke of the contact bridge with the movable contact pieces. Moreover, the contact pressure between the movable and immovable contact pieces need not correspond to the actuating force which acts upon the plunger. For this reason, the known contact devices are provided with a pressure spring which returns the plunger after its actuation and holds it in the starting position, or a contact pressure spring is arranged in each recess of the plunger with each contact bridge. These springs determine and exactly adjust the desired contact pressure. Each contact bridge is held under the pressure of the contact pressure spring which is movable in the recess of the plunger.
This arrangement, however, possesses the following disadvantage: during the switching process electric arc or local overheating on the contact points between the movable and the immovable contact pieces can take place, which leads to adhering or local welding of the contact pieces. During actuation of the plunger one or another movable contact piece can be caught, and either the contact does not open at all or opens only after the travel of the contact bridge over a greater part of the working stroke. As a result of this malfunction the contact bridge becomes inclined and can cause tearing off of the adhered or welded points. In view of these considerations, the known contact devices do not have a sufficient switching reliability.
In German publication DE-B No. 1 935 225 there is disclosed a contact device which is formed as an electrical snap switch. In this construction, a turning lever extending at both sides of the actuating plunger is rotatably supported by pins in the housing of the snap switch. The ends of a turning lever extending toward the actuating plunger can engage in depressions of the actuating plunger, and the depressions are limited by inclined surfaces. Turning of the turning lever is performed so that the ends of the turning lever during actuation of the actuating plunger are first moved into the depressions, and then abut against the inclined surfaces. As a result of the thus produced turning movement, the other ends of the turning lever come into contact with the contact bridge. During further movement of the actuating plunger, the contact supports of the contact bridge are separated from the fixed contact pieces. In the event of adhering or welded contacts a forced separation is performed. The forces required for the forced separation act upon the movable contact parts relatively close to the contact point. Because of the support of the turning lever by means of fixed pivot pins in the housing, a movement reverse takes place or in other words the lever ends acting on the contact bridge are moved in a reverse direction to the actuating plunger and the contact bridge is displaced in the reversed direction. This reversal of the movement direction is in agreement with the movement conditions of the snap switch and is achieved by the construction of the contact bridge of three parts including a central tongue and two outer tongues. The reversal of the movement direction is however also performed in the case of a rigid contact bridge. Moreover, in the known construction a considerable movement play is provided and required, until it comes to a forced separation of the contact. Thereby, unfavorably great inclined positions of the contact bridge at one or other end are produced. Finally, a forced separation of the contact is provided only for the contact breaker contacts and not for the later opening of the contact maker contacts.
A further contact device is disclosed in the EU-A No. 005 0675. In this construction the structural members are seated on the contact bridge parts extending beyond the recess of the plunger and formed by outwardly trapezoidally reducing blocks which are turnable in a small angular region relative to the direction of displacement of the plunger. A projection is provided on the inner side of each block and engages in the recess. The inclined surfaces of the trapezoidal block cooperate with the inner edges and/or inner inclined surfaces of the housing so that each projection after a part of the displacement path of the plunger disengages from the recess. This contact device has the advantage that the forced opening of the contact is practically performed without any delay, or in other words, without previous great movement play. It is further achieved here that the respective movement direction of each contact bridge corresponds to the movement direction of the plunger.